Method for changing the optical characteristics of an article



United States Patent 3,515,587 METHOD FOR CHANGING THE OPTICALCHARACTERISTICS OF AN ARTICLE Eugene C. Letter, Penfield, N.Y., assignorto Bausch & Lomb Incorporated, Rochester, N.Y., a corporation of NewYork No Drawing. Continuation-impart of application Ser. No. 706,700,Feb. 19, 1968, which is a continuation-in-part of application Ser. No.355,959, Mar. 30, 1964. This application Dec. 27, 1968, Ser. No. 787,547

Int. Cl. B44d J/18 US. Cl. 117-217 4 Claims ABSTRACT OF THE DISCLOSURE Asubstrate is coated with a thin film of an alkali halide such as sodiumchloride. Predetermined portions of the alkali halide film are coloredby bombardment by an electron beam or a glow discharge to producecolloidal color centers. A further feature includes an electronconductive film between the alkali halide film and the substrate toimprove color contrast.

CROSS-REFERENCES TO RELATED APPLICATIONS This application is acontinuation-in-part of my copending application Ser. No. 706,700, filedFeb. 19, 1968, and now abandoned, which application was acontinuation-in-part of my prior application Ser. No. 355,959, filedMar. 30, 1964, and now abandoned BACKGROUND OF THE INVENTION Thisinvention relates to a new optical element and a method for changing theoptical characteristics of the element, and in particular to a novelmethod for changing the color in predetermined areas on a surface of acoated article.

It has been determined that it would be desirable to be able to producenew articles comprised of a substrate such as glass coated with a thinfilm, the color of the film changeable by exposure to electron beams.Such a method can be used to produce, among other things optical scales,reticles, gratings, thin film circuits, filter elements, electronsensitive films, photosensitive elements, as well as devices for datastorage and data recording.

Electron beams have low deflection inertia, high energy, highresolution, and are relatively flexible, i.e., they may be deflected andcontrolled easily and rapidly by electrical signals. In addition, theirenergy density is relatively high so that writing times are relativelyshort. Therefore, it is desirable to incorporate electron beams forrecording picture elements or information bits. One approach toutilizing electron beams for recording purposes is disclosed in thearticle, Thermo Plastic Recording, by William E. Glenn and J. EdmundWolfe, which appeared in International Science and Technology, June1962, at pages 2835.

Studies of ionic crystals under electron bombardment indicate that smallcrystals of the alkali halides produce colors when the crystal issubjected to X-rays, cathode rays, and also by heating above 600 C. inan atmosphere of an alkali metal.

Coloration of a halide fihn is the operative mechanism in the well-knownskiatron tube as typified by US. Pat. No. 2,545,200 granted Mar. 13,1951 to G R. Fonda. However, in this type of tube the image does notlast and fades shortly after projection onto the screen. Such a devicecannot be used for recording or information storage as the image is notof a permanent nature. Images of this type are generally not sharp atthe edges and therefore, this teaching would not be applicable to makinga precision reticle or ruling.

SUMMARY OF THE INVENTION In order to produce the articles as disclosedabove, I have found that when a suitable substrate such as glass iscoated with a thin film of an alkali halide, the film or predeterminedareas thereof, may be colored by exposing those areas to electronbombardment. The electron bombardment may be achieved by a controlledbeam of electrons or by glow discharge. In the latter case thepredetermined areas would be exposed through a suitable mask such as aphotoresist. The preferred method of practicing the invention utilizes abeam of electrons in the order of 1 to k v. to produce the desiredcoloration.

Accordingly, it is the primary object of the present invention toprovide a method for producing a color change in portions of a coatedsubstrate.

It is a further object of the present invention to provide a method forproducing a color change in an alkali halide film deposited on a glasssubstrate.

-It is another object of the present invention to provide a method ofcontrolled coloration in an alkali halide film deposited on a substrate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the practice of theinvention a substrate such as glass is cleaned and placed in a vacuumchamber. An alkali halide such as sodium chloride is placed in amolybdenum or tungsten boat. The use of a small boat requires the use ofa current of about 20 to 50 amps to obtain the 10-20 volts used forevaporating the sodium chloride. The pressure in the chamber is reducedto a pressure of less than 1 1O- mm. of mercury and preferably to apressure in the range of l l0 to 1 10- mm. of mercury. An electricalcurrent is passed through the boat to raise the temperature of thesodium chloride whereby a thin film of sodium chloride will be depositedonto the substrate. It is believed that a thin film of onequarterwavelength of light measured at approximately 5500 A. units producessatisfactory results. Other thicknesses will be desirable for differentapplications. At this thickness the sodium chloride film is transparentin order to provide the contrast between those areas containing reducedcolloidal color centers and the rest of the layer.

It is desirable for some purposes to deposit an opaque, semitransparentor transparent film of a metal or conductor such as aluminum, copper,silver, gold, tin oxide, etc., onto the substrate prior to depositingthe sodium chloride. The aluminum film has been found to improve thecolor contrast for various operations. I have found that by grounding(relative to the negative electron beam) the thin conducting film therate of coloration increases and the contrast is improved. The aluminummay be deposited under similar conditions to those used for forming thesodium chloride film. Accordingly, the use of multiple sources allowsfor multiple depositions to be made without breaking the vacuum.

It is known in the prior art to use a conducting film such as I use in askiatron type tube for decreasing the erasure time. This is typified byUS. Pat. 2,836,754 granted to F. Holborn et al. on May 27, 1958.However, I find the film does not only not cause the image to fade, butrather deepens the contrast and improves the image quality.

The conducting film must be placed between the halide film and thesubstrate in order to ground the electrons. The electrons must strikethe sodium chloride film in order to produce the reduced colloidal colorcenters. If the conducting film was placed between the sodium chlorideand the electron source and made conductive to the electron beam thegrounding would not take place. Conversely, if the conducting film wasopaque to the electron beam and on top of the sodium chloride film thecolloidal color centers would not be formed. -It is therefore imperativethat the conducting layer be placed between the substrate and the sodiumchloride film.

The color of the alkali halide film may be changed by irradiating thefilm. Irradiation can be accomplished by a glow discharge at a pressureof about 1-20 microns of mercury. The glow discharge for a non-directedbeam was a D-C glow from i to 5 kv. and -180 milliamps. Using acontrolled beam of electrons it is necessary to have a beam of electronin the order of l-lOO kv. to produce the desired color change.

The color centers are produced by a method of reduction using low energyelectron beams. Using this method the color produced in the sodiumchloride film is in the blue region of the visible spectrum. Adiscussion of the theory underlying the formation of colloidal colorcenters is contained in Color Centers in Solids, by Schuman and Compton,1962, pages 256 to 273.

I have found that a device a described above can be used to measure thedegree and uniformity of a glow discharge in a vacuum coating chamber.Heretofore there has been no reliable method for measuring the intensityof discharge across a substrate to be vacuum coated. Placing a glasssubstrate coated with a sodium chloride film, similar in size to thearticle to be coated, in a vacuum chamber in the position where thepiece to be coated is normally placed and exposing this piece to theconditions (e.g., pressure, temperature, time and intensity ofirradiation) to be used for vacuum coating will cause coloration of thefilm.

The test sample can then be removed and examined for variations in colorintensity across the film. The variations in color intensity will denotevariations in the intensity of discharge, other conditions remainingconstant, to be expected when an actual coating run is taking place.Based upon this information corrections in the vacuum coating cycle canbe made.

Having thus described my invention by reference to a preferredembodiment I wish it understood that it may be modified or embodied inother forms without departing from the spirit and scope of the appendedclaims.

I claim: 1. An optical element comprising a substrate and a thin film ofalkali halide supported by said substrate, a

4 thin electrically conductive film separating said substrate and saidalkali'halide film,

said substrate being glass,

said alkali halide film being sodium chloride, and

said thin conducting film selected from the group consisting of tinoxide, copper, aluminum, gold, and silver.

2. An optical element according to claim 1 in which the sodium chloridefilm has a thickness of about onequarter of a wavelength of lightmeasured at about 5500 A. units.

3. An optical element according to claim 1, wherein the alkali halidefilm contains reduced colloidal color centers.

4. A method for changing the optical characteristics of an articlecomprising the steps of providing a glass substrate,

coating said substrate with a conducting film selected from the groupconsisting of tin oxide, aluminum, copper, gold, and silver,

grounding said conducting film,

depositing a sodium chloride film onto the conducting film, and

exposing the sodium chloride film to a controlled beam of electrons at1-100 kv. to thereby produce colloidal color centers in the sodiumchloride film.

References Cited UNITED STATES PATENTS 2,532,971 12/1950 Van Leer et al.2,533,381 12/1950 Levy et al. 2,545,200 3/1951 Fonda. 2,836,754 5/1958Holborn et al. 2,903,378 9/ 1959 Rychlewski.

OTHER REFERENCES Wikkenhauser, G.: The Skiatron or Dark Trace Tube, inElectronic Engineering, 20 (239), pp. 20-22, January 1948.

Honig, I. M.: Imperfections in Crystals, in Journal of ChemicalEducation, 34, No. 7 (1967), pp. 343-347.

ALFRED L. LEAVITT, Primary Examiner C. K. WEIFFENBACH, AssistantExaminer US. Cl. X.R.

